Hypochlorite (OCl−) and its protonated form hypochlorous acid (HOCl) has been well known and put to commercial uses (e.g., whitening agent and oxidants) with great success since it was first discovered in 1787. In 1825, the use of calcium hypochlorite for the general sanitation of sewers, privies, morgues, hospital wards, ships, and prisons was reported. Further, since sodium hypochlorite was found to be effective against disease-causing bacteria by the end of the nineteenth century, hypochlorite has been widely used as a universal disinfectant for more than 150 years.
In living organisms, hypochlorite can be synthesized in vivo from hydrogen peroxide and chlorine ions in a chemical reaction catalyzed by the enzyme myeloperoxidase (MPO), which may be secreted by activated phagocytes in zones of inflammation. As a nucleophilic non-radical oxidant, hypochlorite can be used as a microbicidal agent (Thomas, E. L., Infect. Immun., 1979, 23, 522-531). Furthermore, neither bacteria nor normal healthy cells can neutralize its toxic effect because they lack the enzymes required for its catalytic detoxification (Lapenna, D. and Cuccurullo, F., Gen. Pharmacol., 1996, 27, 1145-1147).
Generally, hypochlorite can react with some proteins that may play important roles in killing bacterial cells and/or human diseases (Thomas, E. L., Infect. Immun., 1979, 23, 522-531; McKenna, S. M. and Davies, K. J. A., Biochem. J., 1988, 254, 685-692; Hazell, L. J. and Stocker, R., Biochem. J., 1993, 290, 165-172; Hazell, L. J., van den Berg, J. J. and Stocker, R., Biochem. J., 1994, 302, 297-304). When contacting with proteins, hypochlorite may cause damages to the proteins. For example, hypochlorite may alter protein structures, and/or cause fragmentation and dimerization of proteins. As a strong oxidant, hypochlorite can also oxidize low-density lipoproteins (LDL) rapidly. Furthermore, the reaction of hypochlorite with DNA can also result in both chemical modifications and structural changes in DNA (Hawkins, C. L. and Davies, M. J., Chem. Res. Toxicol., 2002, 15, 83-92; Prutz, W. A., Arch. Biochem. Biophys., 1996, 332, 110-120; Arch. Biochem. Biophys., 1998, 349, 183-191; Arch. Biochem. Biophys., 1999, 371, 107-114).
Because of the above-mentioned uses and roles of hypochlorite and its conjugate acid, i.e., hypochlorous acid, there is a need for methods that detect and measure, directly or indirectly, hypochlorous acid and hypochlorite, including in vivo detection and measurement.